Resonant Circuit-Based Vascular Monitors and Related Systems and Methods

Abstract

Systems and methods for control and signal processing in variable inductance, resonant circuit vascular monitoring devices including use of sensor signal magnitude for determining and interpreting sensed parameters are disclosed.

1 . A method for controlling a wireless resonant circuit sensor, the sensor including a variable inductance coil that changes resonant frequency in response to a change in a monitored physical parameter and produces a ring-back signal with a signal magnitude correlated to the physical parameter when energized, the method comprising:
 outputting an excitation signal selected to produce the ring-back signal from said sensor;   receiving the ring-back signals from said sensor at a receiving amplifier;   comparing the magnitude of the sensor ring-back signal to a dynamic range of the receiving amplifier; and   reducing receiving amplifier gain when compared magnitude is at or exceeds a dynamic range of the receiving amplifier.   
     
     
         2 . The method of  claim 1 , wherein said reducing receiving amplifier gain comprises reducing said gain to be within a linear range of the receiving amplifier. 
     
     
         3 . The method of  claim 1 , wherein said reducing receiving amplifier gain comprises reducing said gain when ring-back signal magnitude is increasing and reaches a predetermined magnitude threshold. 
     
     
         4 . The method of  claim 1 , wherein:
 the receiving amplifier comprises an amplifier circuit including an analog-to-digital converter (ADC); and   a receiver gain control function is implemented to reduce receiving amplifier gain when a peak value of the amplified response signal from said sensor reaches a count corresponding to a predetermined percentage of a maximum ADC count.   
     
     
         5 . The method of  claim 1 , further comprising adjusting receiver amplifier gain in response to a detected sensor resonant frequency based on a predetermined map of sensor frequency to gain. 
     
     
         6 . The method of  claim 1 , further comprising:
 adjusting the received ring-back signal magnitude by a transmit efficacy function to provide an adjusted ring-back signal magnitude;   comparing the adjusted ring-back signal magnitude to a physical parameter—magnitude correlation to determine a value for the physical parameter based on said correlation.   
     
     
         7 . The method of  claim 6 , wherein the transmit efficacy function is calculated as equal to 
       
         
           
             
               
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         where f rx  is the sensor resonant frequency and f tx  is the energizing signal transmit frequency. 
       
     
     
         8 . The method of  claim 6 , wherein the transmit efficacy function determined based on an empirically derived transmit frequency efficacy curve. 
     
     
         9 . The method of  claim 1 , further comprising:
 determining physical parameter versus magnitude data for at least one said sensor prior to placement in a patient;   creating a magnitude versus physical parameter characterization curve for the at least one sensor based on said physical parameter versus magnitude data through curve fitting or interpolation;   taking a measurement with the sensor; and   translating the sensor measurement into a value for the physical parameter using said characterization curve.   
     
     
         10 . The method of  claim 9 , wherein the at least one sensor comprises a sensor batch and the magnitude data comprises batch specific parameter versus magnitude data. 
     
     
         11 . The method of  claim 9 , further comprising minimizing physical parameter measurement error arising from sensor manufacturing variability through use of sensor or sensor batch specific characterization curves. 
     
     
         12 . The method of  claim 1 , wherein the resonant circuit sensor is configured for placement in a patient's vasculature and the physical parameter is a vascular dimension. 
     
     
         13 . The method of  claim 12 , wherein said sensor is specifically configured for placement in a vena cava and the vascular dimension is the area of the vena cava. 
     
     
         14 . The method of  claim 13 , further comprising correlating the measured area of the vena cava to patient fluid status. 
     
     
         15 . A method for characterizing a resonant circuit sensor to correlate sensor output to a measured physical parameter, wherein said sensor comprises a variable inductance coil that changes resonant frequency in response to a change in the physical parameter by producing, when energized, a ring-back signal having a signal magnitude correlatable to the physical parameter, the method comprising:
 determining physical parameter value versus signal magnitude data over a range of parameter values and signal magnitudes for at least one said sensor prior to placement in a patient; and   creating a signal magnitude characterization curve for the at least one sensor by plotting a curve with said signal magnitude data using curve fitting or interpolation techniques.   
     
     
         16 . The method of  claim 15 , wherein the physical parameter is an internal vascular lumen dimension comprising area of the lumen, said sensor being implantable within a vascular lumen and expandable and contractable therewith, wherein t said determining comprises sequentially placing the sensor in a series of progressively larger or smaller tubes of known dimension and recording the corresponding ring-back signal magnitudes when energized in each different-sized tube. 
     
     
         17 . The method of  claim 16 , further comprises:
 during manufacture, determining a vascular dimension versus signal magnitude data set for each sensor in a sensor batch; and   creating the characterization curve from the senor batch dimension-magnitude data through curve fitting or interpolation prior to sterilization of the sensors.   
     
     
         18 . The method of  claim 15 , wherein the ring-back signal further includes a frequency correlatable to the physical parameter, and said method further comprises:
 determining physical parameter value versus frequency data over a range of parameter values and frequencies for said at least one said sensor prior to placement in a patient;   creating a frequency characterization curve for the at least one sensor by plotting a curve with said frequency data using curve fitting or interpolation techniques; and   correlating sensor output for the at least one said sensor with the measured parameter based on both signal magnitude characterization and frequency characterization.   
     
     
         19 . A method for controlling a wireless resonant circuit sensor, the sensor including a variable inductance coil that changes resonant frequency in response to a change in a monitored physical parameter and produces a ring-back signal at a frequency or magnitude correlated to the physical parameter when energized, the method comprising:
 outputting a sensor energizing signal at an initial transmit frequency;   receiving the ring-back signal at a ring-back frequency from the sensor in response to the sensor energizing signal;   determining a difference between the transmit frequency and the ring-back frequency;   periodically repeating said outputting, receiving and determining while said difference between the transmit frequency and ring-back frequency is below a predetermined threshold;   changing the sensor energizing signal transmit frequency to a new transmit frequency matching the ring-back frequency of a last received ring-back signal when said difference meets or exceeds the predetermined threshold; and   periodically repeating said outputting at the new transmit frequency and thereafter repeating said receiving and determining.   
     
     
         20 . The method of  claim 19 , wherein said outputting a sensor energizing signal at an initial transmit frequency comprises:
 outputting at least one sensor energizing signal frequency sweep comprising a preestablished number of transmit pulses at pre-defined frequencies over a range of expected sensor resonant frequencies;   receiving the ring-back signals for each of the sequentially output transmit pulses;   transmitting at least one initial transmit pulse for a predetermined initial period, wherein the at least one initial transmit pulse comprises one of—
 a pulse frequency corresponding to the highest amplitude ring-back signal received from the at least one frequency sweep; or 
 plural said energizing signal frequency sweeps; 
   receiving plural test ring-back signals in response to at least one initial transmit pulse transmitted over the initial period;   identifying an initial ring-back signal corresponding to a preferred energizing signal pulse frequency;   selecting said preferred energizing signal pulse frequency as a measurement transmit pulse frequency; and   outputting said sensor energizing signal at the initial frequency as measurement transmit pulses at the measurement transmit pulse frequency for a subsequent measurement period.   
     
     
         21 . The method of  claim 19 , wherein said predetermined threshold is a numerical value. 
     
     
         22 . The method of  claim 21 , wherein said numerical value is 25 kHz or greater. 
     
     
         23 . The method of  claim 19 , wherein said predetermined threshold is a transmit efficacy function. 
     
     
         24 . The method of  claim 23 , wherein said transmit efficacy function is 0.7 or less.